The present invention relates to a method and an arrangement for a receiver in a multi-mode mobile radio, which is adapted to be used in different radio communication networks (e.g. GSM, AMPS).
According to prior art a multi-mode radio mobile telephone, also called a multi-mode phone, is a radio mobile phone adapted to be used in several different cellular radio telephone communication networks, also called cellular networks, which utilises different frequency bands. Examples of cellular networks are AMPS, D-AMPS, GSM, DCS1800, GSM1900, IS-95, NMT and DECT. The different frequency bands result in that the different cellular networks have different carrier frequencies which are used in the communication between the multi-mode phone and the cellular network.
An advantage with the multi-mode phone is that the phone can choose the cellular network which at the present moment has the best radio coverage area and which has the best signal quality. The user of the multi-mode phone can then move from a office network like DECT inside a building to an area outside which is covered by another cellular network. The multi-mode phone is a very good alternative in areas with large traffic density, where an extra cellular network can be used as a complement to an already existing cellular network to increase the capacity.
The receiver in a cellular phone is implemented differently for different cellular networks so, according to prior art, one method to construct a multi-mode phone is to implement separate receivers in the multi-mode phone for the different cellular networks.
A problem with this method is that the separate receivers take big space in the mobile phone, more electronic components are included in the mobile phone and therefore the phone becomes more expensive and weighs more.
The receiver in a radio mobile telephone reconstructs the modulating signal related to the incoming radio frequency signal in the antenna of the radio mobile telephone. The receiver rejects unwanted signals and amplifies the wanted incoming radio frequency signal in a background of noise that lay in the same channel bandwidth as the incoming radio frequency signal. This is well known to the person skilled in the art.
In a traditional receiver, the incoming radio frequency signal is translated in frequency to an intermediate frequency (IF) or a baseband frequency signal where the relevant information is recovered by some demodulation means.
The demodulation signal processing in the receiver of the incoming radio frequency signal is traditionally done by analog circuits (for example an FM discriminator), but today digital circuits are also being used for demodulation, for example in a receiver of a heterodyne structure or a homodyne structure (also called a direct conversion structure). Because of the demand for significant increases in traffic capacity over the same frequency spectrum currently used in cellular networks, requirements for lower cost operation, and additional mobile telephone features, such as battery-saving capabilities, a transition is being made today from analog to digital communication in cellular networks.
For analog cellular networks, like AMPS, ETACS, and NMT, the receiver in a radio mobile telephone is most commonly implemented as a superheterodyne structure. The superheterodyne receiver mixes down an incoming radio frequency signal to an intermediate frequency band and thereafter down to a second lower intermediate frequency. In this receiver, the channel bandwidth selection is made in the intermediate frequency bands with SAW and/or ceramic bandpass filters having a fixed bandwidth.
For digital cellular networks, like GSM, GSM1900, and D-AMPS, the receiver in a radio mobile telephone is normally implemented in a different way, where the receiver uses a lot of digital processing of the incoming signal. This is briefly described below.
This digital cellular network receiver is normally of a heterodyne structure, but could also be of a homodyne structure, which have the same basic principles.
The homodyne receiver mixes down an incoming radio frequency signal in the radio mobile telephone immediately to two baseband channels in quadrature (I and Q).
The heterodyne receiver mixes down the incoming radio frequency signal to an intermediate frequency band before mixing down to two baseband channels in quadrature. The receiver can mix down the incoming signal in several steps before mixing down to the two baseband channels in quadrature. The channel bandwidth selection could be made in three different places; in a bandpass filter at the intermediate frequency, in low-pass filters on the analog baseband signals, and in digital low-pass filters operating on the digitized baseband signals. Digital filtering is of course only possible if the sampling ratio is much higher than the Nyquist sampling frequency (twice the channel bandwidth).
The well known characteristics of a digital filter, is that it is only related to a sampling frequency and not to certain frequency characteristics. If the sampling frequency is changed, then the frequency characteristics of the filter will be scaled accordingly. So for example, if the sampling frequency is changed to half its value, then the bandwidth of the digital filter will be scaled to half its original value.
The patent application EP 678,974 describes a dual frequency transmitter or receiver in a portable phone, in which the receiver includes a circuit switch which switches the antenna in the phone between two frequency front-end parts depending on which frequency network is used. The transmitter or receiver uses two different oscillator frequencies to handle radio signals from both frequency networks at one intermediate frequency. The channel bandwidth remains the same for the two operating frequency networks and thus the channel filtering is the same for the different frequency networks.
The patents U.S. Pat. Nos. 5,008,925, 4,972,455, 5,020,093 and 5,020,092 describe dual channel bandwidth receivers designed for the networks AMPS and N-AMPS (narrow band AMPS) applications where the channel bandwidths are 30 kHz and 10 kHz, respectively. There are two different ceramic channel filters on a second intermediate frequency with different bandwidth. The channel bandwidth is chosen by a switch that connects the signal through one of these filters depending on which network is to be used.
A problem with this solution is that it adds a lot of analog components, such as ceramic filters, which are relatively expensive and not possible to integrate in silicon.
The U.S. Pat. No. 5,369,785 describes an invention that relates to the detection of a signalling tone in a cellular network. A bandpass filter is used for the detection and the center frequency of the bandpass filter is set to the frequency of interest by varying the frequency of an external clock. The actual bandwidth of the filter is not related to the clock frequency.
The problem dealt with by the present invention is to design a receiver in a multi-mode mobile radio that uses the same hardware to process several different channel bandwidths.
Another problem dealt with by the present invention is to design a receiver in a multi-mode mobile radio that uses the same hardware but with some programming of a digital part of the receiver to process several different channel bandwidths.
One intention of the invention is consequently to design a receiver in a multi-mode mobile radio that uses the same hardware or that uses the same hardware but with some programming of the digital part of the receiver to process several different channel bandwidths.
The problem is solved essentially by changing a sampling frequency which controls both analog to digital converters and a digital filter unit in the digital part of the receiver. Thereby different frequency bandwidths can be processed in the same hardware.
The digital filter unit could for most radio communication networks remain constant (i.e. the same filter coefficients and the same filter structure can be used in the digital filter unit) because of the fact that the specified filtering requirements on the adjacent channels are basically the same for all networks. Thus, even for radio communication networks with different channel bandwidths, the specified requirement for the adjacent channel is in the same order.
In some cases, however, the digital filter unit can be modified to a different filter function for adjusting the channel bandwidth of the digital filter unit, as a complement to changing the sampling frequency for achieving a more optimum performance on adjacent channel rejection.
More specifically, the problem is solved in the following manner. An analog part of the receiver in the multi-mode mobile radio is kept the same. There is an exception where the receiver front-end parts (comprising for example low noise amplifier, bandpass filter and/or mixers) are different, if the different networks used by the multi-mode mobile radio operates in different frequency bands.
The channel bandwidth selection is made in the digital part of the receiver by the digital filter unit. By changing the sampling frequency that controls the A/D-converters and the digital filter unit, different channel bandwidths of the receiver are accomplished. The changing of the sampling frequency results in that the bandwidth of the digital filter unit is scaled accordingly. Thereby the desired channel bandwidth at baseband of the radio communication network to be used by the multi-mode mobile radio is set by choosing a corresponding sampling frequency.
As a complement, the digital filter unit can be implemented with a programmable filter function, which programmable filter function comprises a change of parameters and/or filter structure inside the digital filter unit. That is, first the sampling frequency that controls the digital filter unit is changed, resulting in a corresponding change in the bandwidth of the digital filter unit, and then the programmability of the digital filter unit is used to fine tune the filter unit properties. This results in that the desired channel bandwidth of the network to be used by the multi-mode mobile radio is selected in the digital filter unit.
One advantage offered by the invention is that the same components in the receiver are reused. The analog part remains exactly the same in the receiver, except for the fixed frond-end parts in case of several radio communication networks operating in different frequency bands. No modifications of expensive analog components have to be done in the receiver. In some cases a minor modification of the digital filter unit is done, but that is cheap and simple.
Another advantage is that the invention is simple to use in practice as the selection of one of the radio communication networks is made in the digital part of the receiver.
Still another advantage is that the receiver is compact as digital components are compact.
Yet other advantages are that the receiver obtain a low weight as well as low cost.
The invention will now be described in more detail with reference to exemplifying embodiments thereof and also with reference to the accompany drawings.